The Bonneville Power Administration, which sells hydroelectric power to the PUD, is proposing a 9.6 percent increase in its power rates and a 13 percent increase in transmission rates to take effect Oct. 1.

The Bonneville Power Administration is a nonprofit federal utility that supplies power to much of Washington, Oregon, Idaho and Montana, and slivers of Wyoming, Utah, Nevada and California. The agency, based in Portland, Ore., operates 31 dams on Northwest rivers and a nuclear power plant in Richland.

Because the PUD buys some surplus power from Bonneville that it can sell for a profit on the open market, its power-rate increase could amount to as low as 4.4 percent, said Mike Hansen, a spokesman for the federal agency.

Additionally, “We’ll be working with the BPA over the next several months to look at ways it might be able to be reduced,” said Neil Neroutsos, a spokesman for the PUD.

Several factors are spurring the rate increase but two are at the top of the list, Hansen said.

Bonneville sells its leftover power on the open market, but those sales have slowed recently because of the falling price of natural gas, he said. Bonneville, celebrating its 75th anniversary this year, also faces maintenance and repair costs with some of its aging dams, including 71-year-old Grand Coulee Dam in Eastern Washington.

Both the BPA and the PUD will provide chances for public input before any increases are approved, both spokesmen said.

The PUD increased electricity rates last April by 2.9 percent, amounting to $2.56 per month for the average ratepayer.

Transmission costs for additional power purchased from the BPA were part of the reason for the increase. The PUD last year also incurred costs associated with licensing its own Jackson Hydroelectric Project, consisting of Culmback Dam at Spada Lake and a powerhouse downstream.

The PUD buys more than 90 percent of its power from Bonneville, with most of the rest coming from the Jackson project, two mini-dams near Sultan and Monroe, and wind power from east of the Cascades.

The PUD is Bonneville’s largest customer, buying about 10 percent of the federal agency’s output, Hansen said.

The PUD this year is paying $217 million to Bonneville for power and $42 million for transmission, Neroutsos said.